The present invention relates to a holographic recording and reproducing apparatus, and more particularly to a holographic recording and reproducing apparatus capable of removing noise superimposed on a reproduced image upon reproducing data.
A holographic recording and reproducing apparatus that has a large capacity and can perform a high-speed recording and reproducing operation has been paid attention to. When data is recorded in the holographic recording and reproducing apparatus, a signal light spatially modulated on the basis of data to be recorded and a reference light having interference characteristics therewith are applied to a holographic memory as a recording medium so as to intersect together in the holographic memory. Thus, the data is recorded as the interference pattern of light.
The data recorded in the holographic memory in such a way is irradiated with the reference light so that the data can be reproduced. The reference light applied to a prescribed recording position (refer it to as a page, hereinafter) in which the interference pattern is formed is diffracted by a lattice of interference stripes to be formed as a reproduced image. Accordingly, the reference light as the reproduced image is detected by a light receiving unit such as a CCD image sensor to reproduce the data. According to this apparatus, since the image data recorded in the holographic memory is reproduced at a time by allowing the reference light to be incident thereon, a high-speed reproduction can be realized.
Ordinarily, since noise components are included in the data reproduced in the holographic recording and reproducing apparatus, the noise components need to be removed by any of methods. The noise components are originated from various kinds of causes such as an optical system including a light source or a lens, media itself or an input and output device including a spatial light modulator (SLM) or a charge coupled device (CCD).
As a general method for removing noise, there is a method for carrying out an image process or an error correction coding (ECC) process to the detected reproduced image or a method for assuming a common noise pattern to all pages to perform an image process. Further, a method for reducing an influence of noise resulting from a recording medium or noise due to external light is also well-known (see Patent Document 1). In this method, a light beam including an S-polarized light and a P-polarized light is polarized and split. After a signal is supplied to the S-polarized light, both the polarized lights are synthesized to apply the light beam to a hologram recording medium. During reproducing the data, the images of S and P polarized light components are independently detected to obtain a difference signal between both the images. Thus, a common noise component is removed.
[Patent Document 1] JP-A-13-291242
In the above-described usual method for removing the noise, since the S-polarized light is split from the P-polarized light, optical paths need to be respectively provided therefor so that an optical system is undesirably enlarged. Further, in the above-described method, noise caused from a spatial light modulator (SLM) provided on the optical path of the S-polarized light cannot be undesirably removed.